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Particle physics
PARTICLE PHYSICS The picture here shows a cyclotron used to accelerate particles. Particle physics is concerned with the fundamental particles and the interactions between which will be discussed here. There is a long list of fundamental particles, some discovered and some still in theory. These particles interact with the help of some fundamental forces, basically four as we know them arranged in the order of their strength below– 1. Strong Force 2. Electromagnetic Force 3. Weak Force 4. Gravitational Force In physics, every force is supposed to be carried by particles so there are force carriers for all the forces mentioned. Electromagnetic Force Electromagnetic force is the one which is experienced on a wide basis in our daily life. Tension in ropes, frictional force and even the force between an atomic nucleus and an electron are examples of this force. The force carrier of this force is photon. A photon has no mass of its own. It is represented by the greek letter gamma. Gravitational Force This is also a force observed in daily life. The force carrier is called a graviton which is massless. Strong Force The atomic nucleus consists of protons and neutrons bounded together within a radius of approximately 10^-15 m.The protons are all positively charged still they are bound within the nucleus along with other protons. This is because of the action of strong forces. These strong forces are responsible for holding together the nuclei as well as for the building up of protons and neutrons (as we will see). It is carried by a particle called gluon. Weak Force A phenomenon involving weak force is the beta decay. In case of beta decay, a neutron changes into a proton and an electron along with the ejection of an antineutrino. This is in fact a two-step process, first a neutron decays into a proton emitting a W boson which converts into an electron and an antineutrino, W boson ''being the carrier of the weak force. Another force carrier is the ''Z boson. SEEING ON A SMALL SCALE In order to see something on a nuclear scale, we need a very short wavelength. This cannot be provided by electromagnetic radiation as it very difficult to produce such short wavelengths. So, the method adopted is to use the wavelengths associated with the particles to observe matter on a nuclear scale. Since, the higher the speed of the matter particles, the smaller the wavelength associated with them (De Broglie Hypothesis), so, we can look deeper and on a smaller scale. That is need of high energy particle accelerators. Accelerators like cyclotrons use electromagnetic fields to accelerate particles. Particles are detected with the help of detectors like bubble chambers and cloud chambers. SPIN OF PARTICLES Particles have an intrinsic angular momentum or spin measured in the units of Planck’s constant divided by 2 times pi. This spin has no classical analogue. Spin can either be directed up or down. BUILDING BLOCKS OF NATURE ANTIPARTICLES Every matter particle has its anti version which has the same mass, spin, size as the particle but the antiparticle has charge opposite to that of its particle version. For example, there is a particle called a positron which has mass the same as an electron, but, its charge is +e, same as a proton. When these particle and antiparticle combine, they annihilate (collapse) to form energy which may be in the form of gamma ray photons as in the case of electron-positron annihilation. The symbols used for antiparticles may be the same as the particle but with the opposite charge mentioned above it. Sometimes, the term used for a particle may refer both to the particle and the antiparticle, for example, the term ‘neutrino’ may refer to both neutrino as well as antineutrino. FERMIONS Fermions are particles, fundamental or composite, which have a spin of the form: (2n+1)/2.Fermions include protons, neutrons, electrons etc. They follow Pauli’s exclusion principle. Quarks and leptons are examples of fermions. · QUARKS Today, it is known by physicists that even subatomic particles like proton and neutron consist of smaller particles. These particles are known as quarks. They are considered as the fundamental particles, for now(may be when we are able to see on a smaller scale, we can see more fundamental nature). Although, quarks are not separated individually till date but they were found to be the constituents of protons and neutrons by the scattering of electrons. Quarks carry a fractional charge, +2/3 or -1/3 and a spin 1/2. The protons and neutrons are made of up and down quarks. Up quark carries a charge +2/3 and down quark carrying charge -1/3, the down quark having a larger mass than the up quark. Quark spins add or subtract as long as the sum is not negative. So if two quarks combine, then the sum will either be 1 (+1/2 + 1/2) or 0 (+1/2 – 1/2). Similarly, if three quarks combine, they will give a total spin of either 3/2 or 1/2. For a proton as well as a neutron, two quarks have spin in the same direction, the third in the opposite direction resulting in a total spin of 1/2. In a proton, there are two up quarks and one down quark resulting in a net charge of +1 while in a neutron, there are two down quarks and one up quark resulting in no net charge. The reason that three quarks with the same spin are not found together is that they have a slightly higher total energy. These composite particles are known as delta resonances. Other combinations can also be created but quantum mechanics restricts some of them. Although, these quarks have a very little mass but confining them in a small region result in the increase in the energy of the combined particle resulting in higher mass of the protons and neutrons as compared to the constituent quarks put together. A neutron is more massive than a proton because it contains an extra down quark instead of an up quark (down quark is more massive than an up quark) and due to the difference in electrostatic forces between the different quarks present in them. · LEPTONS There are twelve leptons till date including electrons. These particles have a spin ½ and do not feel the strong interaction. They experience the electromagnetic, gravitational and weak force. These particles are not made up of quarks. They are themselves fundamental. Six of them include (rest six are the antiparticles of these leptons)- The neutrinos are particles without any charge but with a small mass which is not possible to measure at present. They are generally produced in decay processes along with the three charged leptons. For example, in the case of beta decay, an electron antineutrino is produced along with an electron. BOSONS All the particles that have an integral spin are bosons. These also may be elementary like graviton and photon or composite like mesons. Elementary bosons include photons, Z and W bosons, gluons, Higgs Boson and gravitons. · MESONS Mesons are particles made of a single quark and an antiquark. Mesons are having a small mass. Examples are pions such as π+ made of an up quark and a down antiquark. These are short-lived because they contain both a particle and an antiparticle. BARYONS Baryons are particles made up of three quarks or antiquarks or a combination of both. Examples include neutron and proton, both of them are made up of three quarks. In the way, electrons in atoms are excited to higher energy states, quarks in hadrons may be in an excited state resulting in more massive hadrons than the original ones. These are known as resonances. HADRONS Hadrons are particles made up of quarks or antiquarks. Thus, mesons and baryons are hadrons. THREE GENERATIONS OF MATTER Nature seems to have divided some particles into three generations. These generations seem to be based on mass. An electron has a heavier version called muon and another yet heavier version, the tau particle. All have the same charge and the same spin. Similar to this are the three types of neutrino, all chargeless and having the same spin. It is found that up and down quarks also have heavier versions of their own. Up quark’s heavier partner is the charm quark and a yet heavier one, the top quark. Similarly, down quark has strange quark as its heavier version and bottom quark in the third generation. (Positive strangeness of a particle is defined as the total number of strange antiquarks and negative strangeness is the number of strange quarks.) STRONG FORCE Similar to the electrostatic charges, there is a new variety of charge carried by the quarks and not by leptons which are responsible for strong forces. Thus, quarks experience strong forces while leptons are blind to them. Quarks carry the new charge in the positive form while antiquarks carry it in negative form. These charges are given colors (this does not mean quarks are colored or they produce colors, it is just a method of distinguishing the charges) red, blue and green. The rule is that unlike colors attract while like colors repel. Antiquarks carry anti-red, anti-blue and anti-green. The attraction between a color and an anticolor being larger than between two colors. Gluons emitted by quarks are the carriers of strong force. An example will be a red-antigreen gluon being emitted by a red quark, which is transformed to a green quark; another green quark absorbing this gluon to form a red quark. QUESTIONS UNANSWERED Where is the extra mass? According to the physicists, the mass of the universe is more than the visible matter. This suggests that the mass is present in some form which neither interacts with electromagnetic radiations nor generates them. This is known as dark matter and large undetected objects formed due to the clumping of dark matter under the effect of gravitation are referred to as MACHOs (Massive Compact Halo Objects). They are now proposed to be consisting of supersymmetric particles. According to Supersymmetry, there exist supersymmetrical partners of all particles. But this theory is not confirmed. How do particles possess mass? This is another question to which Higgs Boson is considered the answer. The whole universe is considered to be pervaded by Higgs field which interacts with some particles to give them mass. If photons do not have mass then they are supposed not to be interacting with the Higgs field. But Higgs Boson is not found till date. Why there is more matter than antimatter? This is also one of the unanswered questions of particle physics. Answers lie in the high energy particle experiments which are supposed to create quark gluon plasma from the composite particles consisting of quarks and gluons in the same way as high temperatures strip electrons out of atoms into plasma. So maybe we see universe in a new way soon.